(1) Field of the Invention
This invention relates to an improved coking process which a highly aromatic oil is mixed with the residual feed passing into a coking zone. In a more specific instance this invention relates to an improved delayed coking process in which slurry oil, which has been decanted, is added to the residual oil feed to the delayed coking process.
(2) General Background
Coking operations in modern refineries produce solid coke, gaseous and liquid products from heavy residual feedstocks.
In the usual application of the coking process, residual oil is heated in a furnace, passed through a transfer line and discharged into either a coking drum or a fluidized coking unit. During coking the residual feedstock is thermally decomposed to a very heavy tar or pitch which further decomposes into solid coke and vapor materials. The vapors formed during decomposition are ultimately recovered from the coking zone and solid coke is left behind.
When a delayed coking operation is utilized, the residual oil is passed into a coking drum which eventually fills with a mass of solid coke. The vapors formed in the coking drum leave the top of the drum and are passed to a fractionating column where they are separated into liquid and gaseous products. Sometimes these products are recycled with residual feed to the coke drum.
In delayed coking operations the residual feed passing into the coking drum is stopped after a predetermined period and routed to another drum. The first drum is then purged of vapors, cooled and opened so the solid coke material which has filled the drum can be removed by drilling or other means.
In fluidized coking a residual feed contacts a previously produced hot fluidized bed of coke particles and is converted to additional coke material and lighter hydrocarbons. The coke in the fluidized bed is heated through external means which include either a gasification zone, where a part of the fluidized coke produced from the residual feed is burned with oxygen, or through heat exchange with a combustor.
In either type of coking operation the refiner generally aims to minimize coke production and maximize liquid products from a residual feed, since liquid products are more easily converted into gasoline or other product of higher value than solid coke.
Applicants have found when a highly aromatic oil containing above about 60 percent of its carbon atoms as aromatic carbons is added to the residual feed passed into the coking unit, reduction of solid coke yield and an increase in liquid yield occur. The addition of the highly aromatic oil in quantities of from about 5 to up to about 20 percent by weight of the total hydrocarbon feedstock being added to the coking unit is optimum, since very large concentrations of highly aromatic oil addition increase the coke yield when compared to additions in the 5 to 20 percent range.
Accordingly, any process improvement in a coking operation which decreases solid coke production and increases valuable liquid production is of interest to a refiner.
One method used to improve coker operations is disclosed in U.S. Pat. No. 3,493,489 (U.S. Class 208-50) where a combination of catalytic cracking and coking is used to increase liquid yields of heavy residual feedstocks. This patent discloses, at column 3, lines 20 through 25, that coker feed material can include the bottoms from the catalytic cracking effluent fractionation column which includes decanted or slurry oil materials. However this patent only generally teaches the use of catalytic cracking effluent fractionation bottoms and does not recognize that a specified percentage of a highly aromatic oil, such as a slurry or decanted oil, combined with the residual feed passing into a coker can produce increased valuable liquid yields while decreasing the coke yield in the coker.
U.S. Pat. No. 3,891,538 discloses the use of a decanted oil, comprising material boiling above about 800.degree. F., which is passed into a coking zone along with residual feedstocks. No specific ratios of decanted oil to coker residual feeds are disclosed or suggested. It is interesting that the claims of this patent require an increased coke yield. Applicants' process results in a reduction in solid coke formation and an actual increase in the C.sub.5 + liquids produced from the coking zone. As illustrated in FIG. 2, depending on the transfer line temperature in the coking operation, addition of highly aromatic oil, such as decanted oil from fluidized catalytic cracking unit, at concentrations above about 25 to 60 weight percent of the feed to the coking unit actually causes the coke yield to increase.
An article by N. P. Lieberman entitled "Shot coke: its origins and prevention," published at pages 45 and 46 of the July 8, 1985, issue of the Oil and Gas Journal, generally describes the use of additions of highly aromatic slurry oil at 5 percent concentrations to a delayed coker to reduce shot coke production.
This article does not recognize that a specific range of slurry oil concentrations can be used to reduce solid coke production and to increase C.sub.5 + liquid yield from residual coker feeds.
In other known coking processes, feeds which comprise essentially 100 percent highly aromatic oil such as hydrotreated slurry or decanted oil produce premium needle coke in the coking operation. Needle coke is an especially valuable and highly specialized form of coke